Pavement markings are used on roadways to display traffic lanes and other traffic information to motor vehicle drivers. Very often pavement markings are retroreflective so that motor vehicle drivers can see the markings at nighttime. Retroreflective pavement markings have the ability to return a substantial portion of incident light towards the source from which the light originated. Light from motor vehicle headlamps is returned toward the oncoming vehicle to illuminate, e.g., the boundaries of the traffic lanes for the motor vehicle driver.
Known retroreflective pavement markings typically include a rubber base sheet that contains pigments and fillers. Optical elements and/or skid-resistant particles are typically secured to a base sheet by being embedded therein or are secured thereto by a bonding material or binder. Pigments and fillers typically are dispersed throughout the base sheet for a number of reasons, including reducing cost, improving durability, and providing conformability. Pigments have also been placed in the bonding material to enhance visibility of the pavement marking and as part of the retroreflective mechanism.
When the pavement marking is retroreflective, it may include a raised pattern of protrusions on the upper surface of the base sheet to provide a more effective orientation for retroreflection and/or to elevate the optical elements above any water or other liquids on the roadway, thereby enhancing reflectivity of the pavement marking under wet conditions; see, for example, U.S. Pat. Nos. 5,227,221; 5,087,221; 5,087,148; 4,988,555; 4,988,541; 4,969,713; and 4,388,359.
As the spacing between the raised patterns of protrusions has been increased to improve retroreflectivity by reducing shadowing effects (see, e.g., U.S. Pat. No. 5,670,227), the susceptibility of the pavement marking to snowplow damage has increased. The damage is thought to be caused by the digging action of the snowplow blade as it falls into the valley areas between protrusions and then strikes the sides of the protrusions.
One approach to improving the resistance of pavement markings to snowplow blades is described in International Publication No. WO 99/25928 (Hedblom et al.) in which elongated and overlapping protrusions are provided to reduce the digging action of snowplow blades.
In addition to, or in place of, changes in the shape and/or patterns of protrusions to improve resistance to snowplows, some pavement markings are also applied in grooves that are formed in the road surface. Typically, a groove about 1.2 millimeters (mm) to about 2.5 mm deep is mechanically formed in the road surface using, e.g., grinding equipment. The pavement marking is then applied within the groove. The ground surface within the groove provides an excellent surface for adhesion of the pavement marking and the depth of the groove provides some protection for the optics on the pavement marking.
There are, however, some disadvantages to this application method, including the cost of forming the grooves in the road surface. The grooves may also provide a point at which the effects of, e.g., the freeze/thaw cycle, may be enhanced, thereby degrading the road surface around and within the groove. In addition, the groove depth may be difficult to control in, e.g., asphalt surfaces. A further disadvantage may be found in the delay between formation of the grooves and application of a marking within the groove.
Furthermore, the groove may collect water which can defeat the purpose of providing raised protrusions to raise the optics above the water to enhance wet retroreflectivity of the pavement marking.